`Clara Oswald is the Dr's current companion. She joined during Mat Smiths era. The same actress has played three separate characters in the series.

This is explained by her jumping into the Dr's timestream when the 'Great Intelligence' was attacking his existance. She travelled into all the Doctor's past incarnations are helped to save the doctor multiple times by stopping the great intelligence from affecting the past.

This is shown as a montage with clara meeting each of the previous doctors. Many of the events the doctor is not even aware of her. In some of the events she is aware of her role, but in others she is not (this is not explained).